1. Field of the Invention
The present invention generally relates to an organic EL (Electro-Luminescence) element suitably used for a display device, a light emission element, etc. of apparatuses for various information industries, and more particularly to an organic EL element, which can maintain a stable light emission property for a long period of time.
2. Description of the Related Art
The EL element is constructed such that an organic luminescent material layer is interposed between a pair of electrodes opposed to each other. By injecting the electrons from one of the electrodes and injecting the holes from the other of the electrodes to the organic luminescent material layer, the electrons and the holes are combined in the organic luminescent material layer, so that the EL element emits the light. The EL element is superior in the visibility and the endurance against a physical shock, and also has a merit of the high variety in colors of emitted light from the organic material forming the organic luminescent material layer, so that it is suitably used for a display device and a light emission element of apparatuses for various information industries, for example.
On the other hand, the organic EL element has such a demerit that, after it has been driven for a certain period of time, the light emission properties such as a light emission intensity and a light emission uniformity are considerably degraded as compared with those at the initial state. One of the reasons for this degradation of the light emission properties is that the moisture, which has been absorbed on a surface of a constitutional part of the organic EL element, and the moisture, which has invaded into the organic EL element, invades through a defect etc. Of the surface of a cathode into the lamination body, which has a pair of electrodes (i.e. the cathode and anode) and the organic luminescent material layer interposed therebetween, so that the detachment between the organic luminescent material layer and the cathode is caused. As a result, a so-called dark spot, i.e. a spot of the light emission portion, which does not emit the light because the electricity is not supplied thereto, is generated.
Therefore, it is necessary to decrease the humidity in the internal of the organic EL element so as to prevent the generation of the dark spots.
In order to prevent the internal of the organic EL element from becoming a high humidity condition, there is proposed one method, which has such a construction that a sealing layer and a protection layer including a drying agent are disposed at the external of the lamination body of the cathode, the organic luminescent material layer and the anode (Japanese Patent Application Laying Open NO. Hei. 7-169567). There is also proposed another method, which has such a construction that the lamination body of the pair of electrodes opposed to each other and the organic luminescent material layer interposed therebetween is accommodated in an airtight case, and that drying means composed of phosphorus pentoxide (P.sub.2 O.sub.5) is accomodated in the airtight case and is spaced from the lamination body, so that the lamination body and the drying means are midair-sealed in the airtight case (Japanese Patent Application Laying Open NO. Hei. 3-261091).
However, the above mentioned organic EL element, which has the construction that the sealing layer and the protection layer including the drying agent are directly laminated on the external of the lamination body (Japanese Patent Application Laying Open NO. Hei. 7-169567), a leak current and a cross-talk can be easily generated because of the formation of the protection layer, so that it considerably degrades the light emission property.
On the other hand, the above mentioned organic EL element, which has such a construction that the lamination body and the drying means composed of P.sub.2 O.sub.5, are accommodated in the airtight case (Japanese Patent Application Laying Open NO. Hei. 3-261091), although it does not have such a problem that the leak current or the cross-talk is easily generated, P.sub.2 O.sub.5, absorbs the moisture in the atmosphere and is dissolved into the absorbed water (deliquesconce), so that phosphoric acid is generated, which badly influences the lamination body. Further, since the method of enclosing the drying means composed of P.sub.2 O.sub.5 into the airtight case is extremely limited, this organic EL element is not practical.